There are various known types of treatment devices, which have a treatment unit that is to be supplied with fluid. The known treatment devices include the blood treatment machines, for example. Various blood purification methods, which are performed in an extracorporeal circulation, have thus become known in kidney replacement therapy, involving the removal of substances that must be eliminated in urine from the blood, i.e., blood constituents, which are eliminated through the kidneys in a healthy person.
Other known extracorporeal blood treatment machines include plasma separation systems as well as adsorbers for liver support therapy or for treatment of sepsis.
In hemodialysis, substances that are present in the blood and are normally eliminated in urine undergo diffusive mass transport through a semipermeable membrane into a dialysis fluid. Mass transport takes place through the semipermeable wall of a dialyzer.
The dialyzer has a blood chamber connected to an extracorporeal blood circulation and a dialysis fluid chamber connected to a dialysis fluid circulation. The blood chamber and dialysis fluid chamber are separated from the semipermeable membrane. To prevent diffusive loss of electrolytes that should remain in the blood, the dialysis fluid has a certain composition of electrolytes in a physiological concentration.
In addition, excess fluid is withdrawn through a semipermeable membrane during dialysis, this withdrawal of fluid being accomplished due to the pressure gradient prevailing on the semipermeable membrane.
On the other hand, a convective mass transport additionally takes place through a semipermeable membrane of a filter in hemofiltration, in which a pressure gradient on the membrane is also the driving force for the mass transport.
To balance a loss of desired blood constituents, the electrolytes lost through the membrane must be replaced by a substitute fluid. A combination of convective transport and diffusive transport is referred to as hemodiafiltration. In the context of the present patent application, with the known methods and dialysis equipment, the concept of dialysis or a dialysis treatment should be understood to include both purely diffusive dialysis and hemodiafiltration. Because of the large amounts exchanged, there is a need for accurate balancing of the fluid withdrawn from the patient and the fluid supplied to the patient over the entire duration of the treatment. Gravimetric and volumetric balancing devices are known from the state of the art.
Balancing devices using balancing chambers using separation elements and balancing devices using exchange vessels are known for volumetric balancing.
A dialysis machine having a balancing device which uses balancing chambers is known from DE 26 34 238 A1, for example. With the known hemodiafiltration machine, the balancing device has a hollow body with a rigid volume, which is subdivided by a movable separation element into a chamber for fresh dialysate and a chamber for spent dialysate. The chamber for fresh dialysate is connected by an inlet line to a source for fresh dialysate and to an outlet line through which fresh dialysate is supplied to the dialyzer. The chamber for spent dialysate is connected via an inlet line from which spent dialysate can flow into the chamber and to an outlet line leading to an outlet for the dialysis fluid. In addition, pumps are provided for conveying both fresh and spent dialysis fluid, and a control unit is provided, which allows filling of the two chambers in alternation. The balancing accuracy here is based on the fact that the chamber for spent dialysate and the chamber for fresh dialysate have the same volume. Balancing chambers are therefore usually manufactured with a low manufacturing tolerance using materials that have good volume constancy.
An alternative method for volumetric balancing is based on the use of exchange vessels. An exchange vessel here can be connected via fluid-carrying lines to a source for fresh dialysis fluid, to an outlet for the spent dialysis fluid, to an outlet into the dialyzer into an inlet from the dialyzer. Shut-off elements or valves are provided in the fluid-carrying lines and can be controlled in such a way that a first fluid circulation is formed in a first work cycle, connecting the fluid source to an outlet via the exchange vessel, and in a second work cycle a fluid circulation is formed, connecting the outlet of the dialyzer to the inlet into the dialyzer via the exchange vessel.
Spent dialysis fluid thus flows from the exchange vessel into the outlet during the first work cycle and is replaced by fresh dialysis fluid from the dialysis fluid source. In the second work cycle, fresh dialysis fluid flows out of the exchange vessel and into the dialyzer and is replaced by spent fluid from the dialyzer.
In the second work cycle, the amount of fluid flowing out of the dialyzer corresponds to the amount of fluid flowing into the dialyzer, which thus ensures balancing.
The simple design of an exchange vessel allows the dialysis fluid circulation or parts of the dialysis fluid circulation to be designed as disposable items. Such disposable items may preferably be produced from PVC or some other plastic.
The inventors of the present patent application have recognized that, in this case, there may be a mistake in balancing when there is not a constant volume at different operating pressures because of the material used in the exchange chamber.
The inventors of the present patent application have also recognized that there is no constancy of the quantities of fluid due to the compression of the gas bubbles present in the dialysis fluid when the operating pressure changes.
The object of the present invention is therefore to provide a balancing device and a method for balancing with an increased precision in balancing.